In a mechanical timepiece movement, the sprung balance regulating system forms one of the most fragile parts of the mechanism, particularly in the event of shocks that can break the pivot-shanks of the balance staff, or even break the balance-spring at the collet-balance-spring join. In order to avoid this risk to the balance staff, the solution that has been proposed for a long time consists in providing shock absorber bearings, such as the “Incabloc” bearing. Such shock absorber bearings also contribute to protecting the balance-spring, but are insufficient when a shock causes a large travel or deflection of the balance-spring and creates tension at the collet-curve join inside the balance-spring. The risk of breakage at this join is greater when the inner terminal curve is of the Grossmann type with a collet of circular contour, i.e. when the first coil of the inner terminal curve has a larger pitch than that of the following coils, meaning that the space between the first coil and the collet is relatively large. The risk of breakage is further increased when the balance-spring is made of a brittle material, such as silicon, monocrystalline quartz or glass, and/or it is made in a single piece at the same time as the collet by micro-machining techniques or photolithography and galvanic growth, as disclosed in EP Patent No. 1 445 670.